Now moral evidence is nothing but a conclusion concerning the actions of men, deriv’d from the consideration of their motives, temper and situation. Thus when we see certain characters or figures describ’d upon paper, we infer that the person, who produc’d them, would affirm such facts, the death of Caesar, the success of Augustus, the cruelty of Nero; and remembring many other concurrent testimonies we conclude, that those facts were once really existent, and that so many men, without any interest, wou’d never conspire to deceive us; especially since they must, in the attempt, expose themselves to the derision of all their contemporaries, when these facts were asserted to be recent and universally known. The same kind of reasoning runs thro’ politics, war, commerce, oeconomy, and indeed mixes itself so entirely in human life, that ’tis impossible to act or subsist a moment without having recourse to it.
[Hume, Treatise 220.127.116.11.] This is similar in part to the hypothetical days of darkness in Section X of the Enquiry, when Hume explicitly argues that it is possible for even a violation of the laws of nature to receive, in his words, "proof from human testimony" if the testimony is sufficiently extensive and uniform, if the phenomenon admits of sufficient analogy as to be not too singular, and if interfering biases -- Hume has religious biases particularly in mind here, since he has just been arguing that they have a pernicious effect on the quality of testimony -- are not involved. Hume doesn't think any historical testimony has reached that point, of course; but as an empiricist he won't rule it out a priori. Any reason for ruling it out would have to come, as in the case with religion, a posteriori. But in general there's no need for historical testimony to reach that level anyway, since most historical testimony does not describe violations of the laws of nature, and therefore has less to do to reach the point of leaving no room for doubt or opposition on the basis of experience, which is all Hume ever means by 'proof'. People who only read Hume superficially and casually often forget that Hume was a historian; he would go on to write a History of England, and he himself doesn't shirk from treating historical evidence as at least on occasion reaching the level of proof.